Preheat the oven to 350F. Line standard muffin tins with paper lines.

Sift together the flours, baking powder and salt. Set aside.

Put butter, sugar and vanilla in the bowl of an electric mixer. Cream on medium-high until pale and fluffy. Add the whole eggs and egg white, one at a time, beating until each is fully incorporated. Scrape down the sides of the bowl as needed. Change the speed to low, add the flour mixture in 2 batches, alternating with the milk. Beat until well combined. Fold in the chopped strawberries by hand.

Fill each paper liner with batter 3/4 full. Bake, rotating the pans half way through, until golden and a toothpick inserted in the middle comes out clean, 20 – 30 minutes. Transfer to wire racks to cool for 15 minutes, then turn cupcakes out onto rack to let cool completely.

To finish, fill a pastry bag fitted with a large open-star tip with the buttercream. Pipe buttercream onto each cupcake, swirling tip and releasing as you pull up to form a peak. Just before serving, top each cupcake with a thinly sliced strawberry.

Puree strawberries in a food processor. Combine egg whites and sugar in the heatproof bowl of a standing electric mixer set over a pan of simmering water. Whisk constantly by hand until mixture is warm to the touch and sugar has dissolved (the mixture should feel completely smooth when rubbed between your fingertips).

Attach the bowl to the mixer fitted with the whisk attachment. Starting on low and gradually increasing to medium-high speed, mix until stiff (but not dry) peaks form. Continue mixing until the mixture is fluffy and glossy, and completely cool (test by touching the bottom of the bowl), about 10 minutes.

With mixer on medium-low speed, add the butter a few tablespoons at a time, mixing well after each addition. Once all butter has been added, scrape down sides of bowl with a flexible spatula and switch to the paddle attachment; continue beating on low speed until all air bubbles are eliminated, about 2 minutes. Add strawberries and beat until combined. Stir with a flexible spatula until the frosting is smooth. Keep buttercream at room temperature if using the same day, or transfer to an airtight container and refrigerate up to 3 days or freeze up to 1 month. Before using, bring to room temperature and beat with paddle attachment on low speed until smooth again, about 5 minutes.